The present invention relates to strake systems for submerged structures, such as columns, pipes or the like, and more particularly to a strake system for a tension leg platform (TLP), spar platform or semisubmersible platform.
Conventional multi-column offshore structures, such as semisubmersible platforms, generally have one or more vertical columns interconnected by pontoons supporting a deck above a water surface. Tendons connected at the lower ends of the columns anchor the offshore structure to the seabed. The submerged support columns are exposed to ocean currents which produce vortex induced motion (VIM) resulting in resonant vibratory stresses that weaken and damages the columns. Typically, the support columns of an offshore structure are cylindrical. Helical fins secured on the peripheral surface of the cylindrical columns are known to reduce vortex formation, thus reducing or eliminating VIM. Helical fins, however, have not been shown to reduce vortex formation about non-cylindrical structures, such as square or rectangular columns.
The economics of recovering oil and gas located offshore at great depths dictate that the construction cost and payload capacity of offshore platforms, such as multi-column submersibles, be optimized. Incorporating non-cylindrical columns in the platform design yields efficiencies in construction and less cost. However, non-cylindrical columns can be as susceptible to vibratory movement as cylindrical columns. This disadvantage may be overcome by mounting strakes on the non-cylindrical columns of the platform in accordance with the present invention which will substantially reduce or eliminate VIM.